Answers

1. "welcome home" or "welcome back" are more common. But if someone has been away for a long time, ou could say, "welcome back home."

2. "pay the bill", or "pay for the meal". But not "pay for the bill."

3. "internet" is not a countable noun. So, "I have a poor internet connection" is fine, or "I've got a poor internet connection at the moment."

4. "concentrated fruit juice" or "concentrated grapefruit tea" or "fruit juice concentrate" or "grapefruit tea concentrate"in the first two, "concentrated" is an adjective. In the second two, "concentrate" is a noun.